The industrial production of conducting coatings on transparent large-area supports, such as vehicle windows, having high optical transmissivity and low electrical resistivity, particularly to enable the transparent supports to be electrically heated, constitutes a problem which has still not been satisfactorily solved.
The oldest known method for making a glass surface conducting consists in depositing a coating of tin oxide (SnO.sub.2) on the glass by chemical means, for example by atomizing a solution containing tin chloride onto the glass heated to 400.degree.-500.degree. C. However, coatings thus obtained are of relatively low conductivity. It is known to considerably increase the conductivity of these coatings by adding to them a dope in the form of an oxide of a higher valency (Sb.sub.2 O.sub.5) or a lower valency (In.sub.2 O.sub.3). However, in spite of the possible improvements, this method of deposition by chemical means presents a certain number of disadvantages, including the toxicity of the products used or the fumes produced and the difficulty of obtaining uniform reproducible deposits.
Other conventional methods involve depositing the conducting oxide or oxide mixture by physical processes. For example, sputtering techniques enable the constituent material (oxide) of a target of practically unlimited area to be transferred onto the transparent support, placed a short distance from this target, by ionic bombardment produced by glass discharge in an inert gas (preferably argon), energized by a d-c voltage or a high-frequency (radio frequency) alternating potential. Like results may be obtained by using the reactive-sputtering technique, in which oxygen is added to the argon atmosphere and a target is used consisting of metal or a metal alloy which is pulverized and oxidized on contact with the oxygen, to deposit a coating in the form of conducting oxides on the transparent support. The main advantages of these sputtering techniques are the obtaining of uniform coatings over large areas, and the ability to control the progress of the process by measuring quantities such as the pressure of the gas or gases, the applied voltage and the electric current. However, these techniques have the major disadvantage of giving a very low deposition rate, of the order of 300-500 A/min, which is incompatible with the requirements of mass production.
When using a radio-frequency sputtering method, the resistivity of the indium oxide may be considerably lowered by doping it with tin oxide, the minimum resistivity appearing to be obtained at a composition of 20 Moles SnO.sub.2 ; 80 Moles In.sub.2 O.sub.3.
Other physical methods for depositing conducting oxide coatings include thermal-evaporation techniques (non-reactive or reactive).
Non-reactive thermal evaporation consists in evaporating the conducting oxide or oxide mixture under vacuum by heating it with an electron beam emitted by an electron gun. The deposition rates thus obtained are very high (of the order of 1 .mu.m/min), but the deposited coating proves to be under-stoichiometric, and thus strongly brown or black-blue in color. Such a coating remains opalescent even after re-oxidation in the presence of oxygen. Reactive thermal evaporation consists in evaporating the metal (or metals) under low-pressure of oxygen, the metal being heated by the same evaporation source. However, the deposition rate is limited in such a method by the maximum allowable oxygen pressure. As the probability of oxidation of the metal atoms in the path between the source and the substrate increases with the oxygen pressure in the enclosure, in theory one should be able to obtain high deposition rates by increasing this pressure. However, considerable increase in pressure is impossible in practice, as the pressure limits the mean free path of the atoms/molecules because of the collisions which arise, and which favor nucleation, i.e. condensation of the material already in the gaseous phase, thus giving rise to pulverulent opalescent deposits. In practice, the maximum tolerable oxygen pressure is of the order of 10.sup.-4 torr, and the deposition rates obtained are of the order of a few A/sec.
It is likewise known that, in addition to the aforementioned oxides, there are other compounds which enable transparent conducting coatings to be obtained, such as cadmium stannate. This compound is generally deposited by radio-frequency sputtering.
Finally, it has been recently proposed to deposit indium oxide on glass by radio-frequency reactive-ion plating. This method consists in disposing a coil connected to a high-frequency generator between the indium source and the substrate support, respectively serving as an anode and a cathode, and then introducing low-pressure oxygen (of the order of 8.times.10.sup.-4 torr) into the enclosure to create a glow discharge, a part of the indium being evaporated by the Joule effect and becoming ionized at the same time by the glow discharge and the high-frequency oscillating field created by the coil, while being accelerated in the direction of the cathode. However, the deposition rates obtained remain low (of the order of 1-3 A/sec) and the indium oxide coatings obtained are of relatively high resistivity (of the order of 1.5.times.10.sup.-3 .OMEGA..cm). An attempt could be made to reduce this resistivity by making under-stoichiometric deposits, but there is no escaping the reduction in optical transmissivity of the coatings resulting from such deposits. Moreover, the use of resistance heating as the source of evaporation has the great disadvantage of limiting evaporation to small-area sources. As this evaporation takes place in a reactive atmosphere, an oxide film also forms on the surface of the molten metal, tending to stop evaporation which can be achieved only at the price of superheating the crucible to eliminate this oxide film. The elimination of this film results in sudden evaporation of the material, making any control of evaporation rate impossible. Moreover, the use of radio frequencies greatly increases the cost of the process, and it remains limited to coating small surfaces. Finally, the absence of a true cathode in proximity to the substrate poses problems for the initiation of deposition, in particular the lack of ionization and acceleration of the ions at the beginning of deposition, which have repercussions on the quality of the substrate-coating interface.
Large-scale deposition of oxide coatings on large-area transparent supports by the aforementioned reactive methods also requires continuous recharging of the evaporation sources. A certain number of recharging systems are already known. For example, a system is known in which the metal-evaporation source consists of a bar of circular section which traverses the bottom of the enclosure in a sealed manner and which is cooled over the larger part of its length and heated by an electron gun only at its top. Such a system is well adapted to the evaporation of metals which have a melting point close to the evaporation temperature. Another system is also known in which the crucible is continuously recharged with the liquefied metal by feeding it with a wire of the metal, brought close to the crucible by a sheath which guides it subsequently to the feed spool on which it is wound. This system is also limited to metals having a melting point not far removed from the evaporation temperature.
These known recharging systems, however, are difficult to apply to metals such as indium or its alloys with tin and antimony, which have a melting point (less than 150.degree. C) far removed from the evaporation temperature (of the order of 1000.degree. C for a vapor pressure of about 10.sup.-2 torr). With such systems, the metal bar or wire melts prematurely, far from the evaporation region.